Gas turbines and other fuel consuming machines typically convert fuel energy into work, and the work may be used to drive an electrical generator, for example. The amount of work produced is dependent upon the fuel consumption rate and control valves are typically utilized to set the rate of fuel delivery to the combustion portion of the machine. For example, a gas flow control valve may be electronically or manually controlled to increase or decrease the fuel flow rate to a combustor in an attempt to meet the load demand of the machine. Under certain conditions, however, the fuel throughput may be limited, and thus the work throughput of the machine may be limited. For example, the turbine fuel supply system pressure may drop requiring a gas flow control valve to open significantly to satisfy the flow rate demand. In this mode of operation, the gas flow control valve may operate in an “un-choked” state, meaning that the fuel delivery rate is sensitive to changes in upstream fuel supply pressure as well as downstream valve outlet conditions. Conventional turbine systems typically operate in a “choked” state because downstream pressure changes due to changes in turbine cycle conditions, or changes in combustor pressure for example, can cause corresponding fuel rate spikes or oscillations. This method of operation is desirable for disturbance rejection so that transients in the upstream (fuel source) pressure and downstream (combustor) pressure cannot interrupt the steady flow of fuel to the turbine.
Conventional gas turbine fuel control systems require a relatively high pressure gas fuel source so that the flow control valves may operate in the choked state. Typically, this fuel source will be at a much higher pressure than is required for the fuel metering system, and an additional pressure-regulating valve is often utilized to regulate the pressure to the gas flow control valve(s) to a desired set point. Typically, this pressure set point is high enough so that the gas flow control valve(s) will always operate choked.
The problem, however, is that sometimes the fuel source is at an insufficient pressure to choke the gas control valves at all load points, even if the pressure-regulating valve is opened fully to minimize pressure drop through the system. Therefore, a need remains for improved systems and methods for unchoked control of gas turbine fuel control valves.